A filtering arrangement

ABSTRACT

A filtering arrangement for a system comprising a power electronics device and an external power storage/discharge element, which power electronics device comprises a power module and a three-phase filter between the AC power terminals of the power module and the supplying AC power grid, and which power module comprises a DC intermediate power bus, and which three-phase filter between the AC power terminals and the supplying AC power grid comprises two three-phase inductors, and which external power storage/discharge element has two DC power terminals which are connected to the DC intermediate power bus of the power electronics device. The filtering arrangement comprises a common mode inductor which is connected between the DC power terminals of the external power storage/discharge element and the terminals of the DC intermediate power bus of the power module.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a National Stage application of International PatentApplication No. PCT/EP2015/080188, filed on Dec. 17, 2015, which ishereby incorporated by reference in its entirety.

TECHNICAL FIELD

The present invention relates to a filtering arrangement for powerelectronics devices connected to power storage/discharge elements. Moreparticularly, the invention is related to a regenerative AC/AC converterwith a connection from its intermediate DC power bus to an external DCvoltage source.

BACKGROUND

A normal method to adjust AC motor shaft rotation speed is to adjust thesupplying voltage magnitude and frequency by a frequency converter (FC).Prevailing FC technology is called PWM (pulse width modulation), whereinthe device is comprised of a rectifier for mains voltage rectification,a filtered DC voltage intermediate power bus and an inverter bridge forgenerating the output voltage. The output voltage consists of pulsesformed by fast power electronic switches, normally IGBTs (insulated gatebipolar transistors). A similar inverter unit as used in motor side mayalso be used in mains side e.g. in cases where the motor may work ingenerator mode and the generated power is desirable to supply back tothe mains. This kind of regenerative system is called as regenerativeAC/AC converter in this document.

An external power supply/storage element may be connected in the DCintermediate power bus of a FC e.g. for a redundant power supply in caseof mains power malfunction. This kind of power element may be e.g. abattery, a photovoltaic power plant, a fuel cell generator etc.

A problem caused by a frequency converter, especially when comprised aregenerative mains bridge connected to a grounded network, may be thecommon mode voltage in the DC intermediate power bus. The fast switchingspeed of an IGBT, in combination with a possible resonance in connectingcables may generate hazardous voltage spikes across the insulation fromthe external power supply/storage element to ground, thus shortening thelifetime of the insulation layers.

The problem may be avoided by using a dedicated transformer between themains power grid and the frequency converter, but this method increasesa considerably amount of the installation costs.

SUMMARY

The object of the present invention is to avoid the problems of priorart by presenting a novel solution, which reduces the stress forinsulations of external power supply/storage elements connected to theDC intermediate power bus of a regenerative AC/AC converter. Theobjective is achieved by what is stated in independent claims, otherpreferred embodiments are disclosed in the dependent claims.

The characteristic feature of the filter arrangement according to thepresent invention is that a common mode inductor is connected betweenthe DC intermediate power bus and the external power supply/storageelement.

In one embodiment of the invention, the filter arrangement comprises atleast a first capacitor arrangement in star-coupling such that the firstterminals of the capacitors are connected to the middle points of a LCLfilter phase inductors and the second terminals of the capacitors areconnected to the first pole of the external power supply/storageelement, either directly or via a filter capacitor. Further, the filterarrangement may comprise a second star-coupled capacitor arrangementwhich is connected respectively as the first capacitor arrangementbetween the middle points of LCL filter phase inductors and the secondpole of the external power supply/storage element.

The effect of the filtering arrangement according to the presentinvention is that it stabilizes the potential of the external powersupply/storage element by reducing potential oscillations between theexternal power supply/storage element and the ground considerably, whencompared to the corresponding potential oscillations at the other sideof the common mode inductor, i.e. between the DC intermediate power busand the ground. Thus the high voltage spikes which may be hazardous tothe insulations of the external devices will be avoided. The technicalsolution according to the present invention, when compared to the priorart technology of using a dedicated mains transformer, is much lessexpensive e.g. due to the fact that the common mode inductor needs onlyto be dimensioned according to the current supplied by the externaldevice, but not according to the mains current of the AC/AC converter.

The invention is defined in more detail in the present description andthe following examples of embodiments. The scope of protection isdefined in the independent claims and the preferred embodiments in otherclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

Below the invention appears a more detailed explanation using exampleswith references to the enclosed figures, wherein

FIG. 1 presents a main circuit of a regenerative AC/AC converter,

FIG. 2 illustrates modulation and voltages in an inverter bridge,

FIG. 3 presents a first embodiment of a filtering arrangement accordingto the present invention,

FIGS. 4A and 4B present other embodiments of a filtering arrangementaccording to the present invention,

FIG. 5A illustrates voltages between an external device connected to theintermediate DC bus of a regenerative AC/AC converter and ground,without a filtering arrangement according to the present invention, and

FIG. 5B illustrates voltages between an external device connected to theintermediate DC bus of a regenerative AC/AC converter and ground, with afiltering arrangement according to the present invention.

DETAILED DESCRIPTION

FIG. 1 presents a simplified main circuit diagram of a known and typicalregenerative AC/AC converter as an example of power device arrangementwherein the filtering arrangement according to the present invention maybe applied.

In the example the converter comprises of an active mains bridge AFE₁,which is able to feed power in both directions between the 3-phase mainsnetwork R, S, T, and the intermediate DC-circuit DC₁. AFE₁ is connectedto the mains via a so-called LCL line filter unit LFU₁, comprising of afirst 3-phase inductor unit LF₁, a second 3-phase inductor unit LF₂, anda capacitor unit C_(F). The inductor units LF₁, LF₂, may comprise eitherone 3-phase inductor (coils wounded around a common core part) or threeseparate 1-phase inductors. Inverter unit INU₁ creates, from the directvoltage circuit DC₁ which is filtered by a capacitor C_(DC1), anadjustable 3-phase output voltage U, V, W, e.g. for supplying an ACmotor.

Both AFE₁ and INU₁ bridges are similar, both consisting of 3 phaseswitches which are able to connect the phase terminal to either poleDC+, DC−, of the DC intermediate circuit DC₁. One phase switch comprisesof upper leg power components (a controllable switch type component,normally IGBT, with an anti-parallel-connected diode) connected to DC+and similar power components in lower leg, connected to DC−. Theoperation principle on how this type of bridge operates is called PWM(pulse width modulation).

FIG. 2 illustrates a known and commonly used PWM modulation method,so-called sine-triangle wave comparison, which is used to determine howthe active IGBT components of PWM-bridges, in this case of AFE₁, arecontrolled. In the method phase specific sinusoidal reference signals(u_(Rref), u_(Sref), u_(Tref)) are compared to a common triangularsignal u_(Δ). When the instantaneous value of the sinusoidal signal ishigher than that of the triangular wave, the upper leg IGBT is turnedon, and vice versa. E.g. at time instant t₁ the value of u_(Δ) exceedsthe value of u_(Rref), which causes that the R-phase switch is turnedfrom the upper position to the lower position, etc.

The lower part of FIG. 2 illustrates the principal waveform of the DCintermediate circuit potential when the mains supply is grounded (groundpotential marked as 0). Between time instants t₁ and t₂, when all phasesof AFE₁ are in lower position, the DC− pole of the intermediate circuitDC₁ is at 0 potential (due to that the sum of mains phase currents andphase current gradients is always 0 when the output side of theconverter is ungrounded). Similarly, between t₄ and t₅ all phases are inupper position causing DC+ pole to stay at 0 potential, and when thephase switches are in different positions the DC intermediate circuitpotential is between these maximum positions as illustrated in FIG. 2.

Due to stray component values, e.g. stray capacitances between maincircuit and grounded frame, and serial stray inductances in currentconductors, the potential changes of DC intermediate circuit is inpractice not as clean as illustrated in FIG. 2, but includes sometransient oscillation with voltage overshoots after each potential step.This phenomenon may be amplified due to resonances, especially at theend of an external device connected by a long cable to the DC circuit,resulting voltage spikes which may be hazardous to insulations.

FIG. 3 presents an exemplary embodiment of a filtering arrangementaccording to the present invention, used for reducing transientoscillation between the electric potential between the main circuit ofan external device B1, connected to the intermediate power bus DC₁₁, andthe ground. The DC power bus DC₁₁ in this example belongs to a similarregenerative AC/AC converter as presented in FIG. 1, but for simplicitythe inverter unit and the capacitor unit C_(F) of the LCL filter are notpresented here.

A voltage regulator (marked as DC/DC in the figure) may be used betweenthe external device B1 and the DC power bus of a converter in order tomatch the voltage levels.

The filtering arrangement comprises a common-mode inductor LCM₁₁,connected between the DC intermediate power bus of the converter (DC₁₁,poles DC+ and DC−) and the DC connectors of the external device (polesB_(DC+) and B_(DC−)). Further, the arrangement comprises a firstcapacitor arrangement C_(M11), consisting of three star-connectedcapacitors such that in each phase the first terminal of thephase-specific capacitor is connected to the connection point ofLCL-filter inductors L₁₁, L₁₂, and the second terminals of thecapacitors are connected to the positive pole B_(DC+) of the externaldevice. Further, the arrangement comprises a second capacitorarrangement C_(M12), consisting of three star-connected capacitors suchthat in each phase the first terminal of the phase-specific capacitor isconnected to the connection point of LCL-filter inductors L₁₁, L₁₂, andthe second terminals of the capacitors are connected to the negativepole B_(DC−) of the external device. The second terminals of thecapacitors can be connected directly to the negative pole B_(DC−) of theexternal device.

FIG. 4A presents another embodiment of the present invention, comprisingonly one star-coupled capacitor arrangement C_(M). It is connectedsimilarly to e.g. C_(M11) in FIG. 3 to the LCL-filter (L₁, L₂) but thestar point S of the capacitors is here connected to both external devicepoles B_(DC+), B_(DC−) via capacitors C₄₁ and C₄₂.

FIG. 4B presents another embodiment of the present invention, comprisingone star-coupled capacitor arrangement C_(M) which is connected to theLCL-filter (L₁, L₂) like presented in FIG. 4A, but here the star point Sof the capacitors is connected only to one of the external device poleB_(DC−) via a capacitor C₄₃. In this embodiment the poles B_(DC+),B_(DC−), of the external device are connected by a capacitor C₄₄.

As in the case of embodiment of FIG. 3, a voltage regulator (marked asDC/DC in the figure) may be used between the external device and the DCpower bus of a converter in order to match the voltage levels.

FIGS. 5A and 5B illustrate the effect of a filter according to thepresent invention, as relative values wherein 1 corresponds the voltageof the intermediate power bus. FIG. 5A illustrates the common modevoltage without filter, i.e. the voltage between DC+ and ground. It canbe seen that the voltage fluctuates very sharply at high amplitude, withpeak to peak value about 2.0 and the maximum spikes reaching about 1.4.FIG. 5B respectively illustrates the common mode voltage with afiltering arrangement according to the present invention, i.e. thevoltage between B_(DC+) and ground (see FIG. 3). Now the voltagefluctuation is very limited, peak to peak value only about 0.2 and thespikes staying below 0.4.

While the invention has been described with reference to the previousembodiment, it should be recognized that the invention is not limited tothis embodiment, but many modifications and variations will becomeapparent to persons skilled in the art without departing from the scopeof the invention, as defined in the appended claims.

What is claimed is:
 1. A filtering arrangement for a system comprising apower electronics device and an external power storage/dischargeelement, which power electronics device comprises a power module and athree-phase filter between the AC power terminals of the power moduleand the supplying AC power grid, and which power module comprises a DCintermediate power bus, and which three-phase filter between the ACpower terminals and the supplying AC power grid comprises twothree-phase inductors, and which external power storage/dischargeelement has two DC power terminals which are connected to the DCintermediate power bus of the power electronics device, wherein: thefiltering arrangement comprises a common mode inductor which isconnected between the DC power terminals of the external powerstorage/discharge element and the terminals of the DC intermediate powerbus of the power module.
 2. The filtering arrangement according to claim1, wherein the arrangement further comprises: a first capacitorarrangement comprising three capacitors in star-coupling arranged suchthat the first terminals of the capacitors of the first capacitorarrangement are connected to the phase-specific middle points of thethree-phase inductors of the three-phase filter and the second terminalsof the capacitors of the first capacitor arrangement are connectedtogether forming a star-point, and wherein the star-point of the firstcapacitor arrangement is connected to the first pole of the externalpower supply/storage element.
 3. The filtering arrangement according toclaim 2, wherein the star-point of the first capacitor arrangement isconnected to the first pole of the external power supply/storage elementdirectly.
 4. The filtering arrangement according to claim 2, wherein thestar-point of the first capacitor arrangement is connected to the firstpole of the external power supply/storage element via a capacitor. 5.The filtering arrangement according to claim 4, wherein the star-pointof the first capacitor arrangement is connected to the second pole ofthe external power supply/storage element via a second capacitor.
 6. Thefiltering arrangement according to claim 4, wherein the poles of theexternal power supply/storage element are connected by a secondcapacitor.
 7. The filtering arrangement according to claim 2, whereinthe arrangement further comprises: a second capacitor arrangementcomprising three capacitors in star-coupling arranged such that thefirst terminals of the capacitors of the second capacitor arrangementare connected to the phase-specific middle points of the three-phaseinductors of the three-phase filter and the second terminals of thecapacitors of the second capacitor arrangement are connected togetherforming a star-point, and wherein the star-point of the second capacitorarrangement is connected to the second pole of the external powersupply/storage element.
 8. The filtering arrangement according to claim1, wherein the arrangement further comprises a voltage regulator betweenthe external power storage/discharge element and the DC intermediatepower bus of the power electronics device.
 9. The filtering arrangementaccording to claim 3, wherein the arrangement further comprises: asecond capacitor arrangement comprising three capacitors instar-coupling arranged such that the first terminals of the capacitorsof the second capacitor arrangement are connected to the phase-specificmiddle points of the three-phase inductors of the three-phase filter andthe second terminals of the capacitors of the second capacitorarrangement are connected together forming a star-point, and wherein thestar-point of the second capacitor arrangement is connected to thesecond pole of the external power supply/storage element.
 10. Thefiltering arrangement according to claim 4, wherein the arrangementfurther comprises: a second capacitor arrangement comprising threecapacitors in star-coupling arranged such that the first terminals ofthe capacitors of the second capacitor arrangement are connected to thephase-specific middle points of the three-phase inductors of thethree-phase filter and the second terminals of the capacitors of thesecond capacitor arrangement are connected together forming astar-point, and wherein the star-point of the second capacitorarrangement is connected to the second pole of the external powersupply/storage element.
 11. The filtering arrangement according to claim2, wherein the arrangement further comprises a voltage regulator betweenthe external power storage/discharge element and the DC intermediatepower bus of the power electronics device.
 12. The filtering arrangementaccording to claim 3, wherein the arrangement further comprises avoltage regulator between the external power storage/discharge elementand the DC intermediate power bus of the power electronics device. 13.The filtering arrangement according to claim 4, wherein the arrangementfurther comprises a voltage regulator between the external powerstorage/discharge element and the DC intermediate power bus of the powerelectronics device.
 14. The filtering arrangement according to claim 5,wherein the arrangement further comprises a voltage regulator betweenthe external power storage/discharge element and the DC intermediatepower bus of the power electronics device.
 15. The filtering arrangementaccording to claim 6, wherein the arrangement further comprises avoltage regulator between the external power storage/discharge elementand the DC intermediate power bus of the power electronics device. 16.The filtering arrangement according to claim 7, wherein the arrangementfurther comprises a voltage regulator between the external powerstorage/discharge element and the DC intermediate power bus of the powerelectronics device.